Interfacial coatings are often applied to fibers in fiber reinforced ceramic matrix or glass matrix composites to control interfacial bonding between the fibers and matrix. By controlling interfacial bonding, such coatings can improve a composite's strength and toughness. In addition to enhancing the composite's mechanical properties, interfacial coatings protect against strong chemical interactions between the fibers and matrix. For composites that will be exposed to elevated temperatures in air, for example in aircraft engines, the fiber coatings also should be oxidation resistant.
An early interfacial coating was a carbon coating applied to silicon carbide (SiC) fibers in SiC fiber reinforced chemical vapor deposited (CVD) SiC matrix composites. Such a coating can be made in situ during composite fabrication or can be applied to the fibers before matrix infiltration. In both cases, the carbon coating enhances the composite's strength and toughness. Carbon interfacial coatings, however, suffer from poor oxidation resistance.
More recently, CVD boron nitride (BN) has been used as an interfacial coating in SiC fiber reinforced CVD SiC matrix composites. Although CVD BN is more oxidation resistant than carbon, it can be difficult to deposit on fibers. At atmospheric pressure, for example, BN deposition often is complicated by homogeneous nucleation, a gas phase reaction that produces a porous, snow-like coating. As a result, BN coatings often are deposited at reduced pressures. The need to use reduced pressures increases the complexity of the equipment used to deposit BN coatings. In addition, BN coatings can reduce the strength of Tyranno.RTM. SiC/TiC fibers (Ube Industries Ltd., Ube City, Japan), SiC fibers, and carbon fibers during fiber coating or when the fibers are introduced into a CVD SiC matrix.
Therefore, what is needed in the industry is an oxidation resistant fiber coating that can be easily applied to fibers at atmospheric pressure and can coat a greater variety of fibers than BN.